This invention relates to an improved transconductor stage which includes an input circuit portion having signal inputs.
The field of application of this invention is particularly, but not solely, related to monolithically integrated time-continuous filters, and the description to follow will make reference to that field of application for convenience of illustration.
As is known, a number of techniques for providing time-continuous integrated filters have been proposed. Among them, those which appear at present to perform best in high frequency environments utilize a transconductor stage as the filter basic block. Most likely, this is attributable to the voltage-to-current conversion being carried out in an open loop within the transconductor, and to the so-called non-dominant pole of the converter not being limited by the frequency unitary gain of the amplification.
In any case, the performance features which are usually expected of a transconductor stage, for high-frequency applications, are basically the following:
maximum amplitude of the signal to be processed; PA1 low noise generation: PA1 low biasing consumption; PA1 high transconductance (gm) value; PA1 high dynamic range of transconductance (gm); and PA1 high gain, in excess of 50 dB.
The prior art has already provided a transconductor stage design which has good performance features closely approximating the above target features. Reference is directed in particular to a transconductor described in Italian Patent Application No. MI91A 003018 by this Applicant.
That Application discloses a transconductance amplifier provided in a mixed bipolar/MOS technology and comprising an input circuit portion having a differential cell with a pair of source-sharing MOS transistors which are connected to an output circuit portion comprising a pair of base-sharing bipolar transistors.
While being in many ways advantageous and substantially achieving its objective, this prior solution has a drawback in that high transconductance values cannot be obtained concurrently with a low current draw and good linearity.
In addition, this prior solution is unsuitable for operation at low power supply values, e.g. of no more than 3 Volts.
A demand definitely exists at present for analog circuits which can operate on just 3 Volts: and yet afford comparable performance levels to circuits operated on 5 Volts. Some solutions have been proposed in the prior art which would seemingly operate even on supply voltages close to 3 Volts; however, under such operating conditions, these solutions have proved ineffective to ensure control of the transconductance gm value over a sufficiently wide range.
The underlying technical problem of this invention is, therefore, that of providing a transconductor stage which has such structural and functional features as to overcome the above-noted drawbacks of prior art solutions.
The present application provides a low-voltage transconductor with two separate differential cells having separate current bias connections. One side of each differential cell is connected to receive an input signal, and the other sides of the two cells have control terminals connected together.. Each cell is connected to both outputs of the transconductor stage.
According to innovative teachings disclosed herein, thereis provided: An integrated circuit transconductor stage, connected to receive power supply voltages which differ by less than 4.5 Volts comprising: first and second differential cells, each comprising first and second transistors having respective first current-carrying terminals thereof connected in common, and a respective current generator connected to provide a fixed current to the connection of said first current-carrying terminals; each said first transistor having a respective control terminal connected to provide a respective input connection; both said second transistors having respective control terminals thereof connected together; said first transistor of said first cell, and said second transistor of said second cell, having respective second current-carrying terminals thereof jointly connected to provide a first current output; and said: second transistor of said first cell, and said first transistor of said second cell, having respective second current-carrying terminals thereof jointly connected to provide a second current output; whereby said outputs provide a differential signal which generally corresponds to the difference in voltage between said input connections.
According to innovative teachings disclosed herein, thereis provided: An integrated circuit transconductor stage, comprising: first and second differential cells, each comprising first and second transistors having respective i first current-carrying terminals thereof connected in common, and a respective current generator connected to provide a fixed current to the connection of said first current-carrying terminals; each said first transistor having a respective control terminal connected to provide a respective input connection; both said second transistors having respective control terminals thereof connected together; said first transistor of said first cell, and said second transistor of said second cell, having respective second current-carrying terminals thereof jointly connected to provide a first current output; and said second transistor of said first cell, and said first transistor of said second cell, having respective second current-carrying terminals thereof jointly connected to provide ,a second current output; and further comprising a low-pass biasing network connected to bias said control terminals of said second transistors to the long-term average value of said control terminals of said first transistors; whereby said first and second outputs provide a differential signal which generally corresponds to the difference in voltage between said input connections.
According to innovative teachings disclosed herein, thereis provided: An integrated circuit transconductor stage, comprising: first and second differential cells, each comprising first and second transistors having respective first current-carrying terminals thereof connected in common, said second transistors having device dimensions which provide higher gain than said first transistors; and a respective current generator connected to provide a fixed current to the connection of said first current-carrying terminals; each said first transistor having a respective control terminal connected to provide a respective input connection; both said second transistors having respective control terminals thereof jointly connected to a variable bias current supply which provides a variable bias current to dynamically adjust the transconductance of said stage; said first transistor of said first cell, and said second transistor of said second cell, having respective second current-carrying terminals thereof jointly connected to provide a first current output; and said second transistor of said first cell, and said first transistor of said second cell, having respective second current-carrying terminals thereof jointly connected to provide a second current output; whereby said outputs provide a differential signal which generally corresponds to the difference in voltage between said input connections.
According to innovative teachings disclosed herein, thereis provided: An integrated circuit transconductor stage for high-frequency filters operated on a low voltage supply, comprising: an input circuit portion having signal inputs; and a pair of differential cells being connected together and each associated with a corresponding signal input, each said cell incorporating at least one pair of transistors having at least a corresponding current-carrying terminal in common; each said cell comprising a respective current generator connected to provide a fixed current to said common terminal.
According to innovative teachings disclosed herein, thereis provided: An analog filter circuit, comprising: a differential input stage, a differential output stage, and one or more additional differential stages, each comprising: first and second differential cells, each comprising first and second transistors having respective first current-carrying terminals thereof connected in common, and a respective current generator connected to provide a fixed current to the connection of said first current-carrying terminals; each said first transistor having a respective control terminal connected to provide a respective input connection; both said second transistors having respective control terminals thereof connected together and to a biasing circuit; said first transistor of said first cell, and said second transistor of said second cell, having respective second current-carrying terminals thereof jointly connected to provide a first current output; and said second transistor of said first cell, and said first transistor of said second cell, having respective second current-carrying terminals thereof jointly connected to provide a second current output; said stages being connected together, with one or more passive elements, in a circuit relationship to implement a desired continuous-time filter relationship from said input connections of said input stage to said outputs of said output stage.